In recent years, in an automobile field, there has been an increasing need for using high-strength steel sheets for a vehicle body, parts, and so on in order to reduce the weight of the vehicle body with the aim of improving fuel efficiency and reducing an emission of carbon dioxide gas (CO2) and to improve collision safety. Meanwhile, in processes of assembling the vehicle body, attaching the parts, and so on, spot welding is mainly used.
As important properties of a joint formed by the spot welding (hereinafter, referred to as a spot-welded joint), tensile strength and fatigue strength can be cited, but what is important first is tensile strength. Tensile strength of the spot-welded joint includes tensile shear strength (TSS) measured under tensile loads applied in shear directions and cross tensile strength (CTS) measured under tensile loads applied in peeling directions.
Generally, as for tensile strength of the spot-welded joint, TSS and CTS with sufficiently high values can be obtained with a good fracture state and little variation of strength when there is no defect or crack in weld metal (nugget) and the weld metal has a good property. Further, as for tensile strength of the spot-welded joint, it is possible to improve CTS also by sufficiently securing its nugget diameter (joint area) and so on.
As a method of improving cross tensile strength of a spot-welded joint formed by spot welding of high-strength steel sheets, there has been proposed a spot welding method in which two-stage welding, two-stage welding including cooling in the middle, or three-stage welding is performed under predetermined conditions at the time of welding (for example, refer to Patent Literature 1).
Further, as a spot welding method that gives high joint strength in a short time, there has been proposed a method including: a main welding step of obtaining a nugget with a predetermined diameter; and a post-heating welding step of repeating suspension for predetermined cycles and short-time welding while sandwiching steel sheets with the same pressurizing force as that in the main welding step (for example, refer to Patent Literatures 2, 3).
Further, as a spot welding method that gives high joint strength, there has been proposed a method including: a first step of forming a nugget in main welding; a second step of cooling a weld while keeping steel sheets sandwiched by electrodes, after the end of the first step; and a third step of passing a higher welding current than that of the main welding within a range causing no expulsion for a short time, after the end of the second step (for example, refer to Patent Literature 4).
Further, as a spot welding method that stably gives high joint strength, there has been proposed a method in which resistance spot welding is performed as welding composed of three first, second, and third stages, the welding in the second stage is performed as welding with a higher pressurizing force, a lower current or the same current, and a longer welding time or the same welding time as compared with the welding in the first stage, and in the third stage, welding with a higher current than that of the second stage is repeated (for example, refer to Patent Literature 5).
Further, as a spot welding method that stably gives high joint strength, there has been proposed a method in which resistance spot welding is performed as welding composed of three first, second, and third stages, the welding in the second stage is performed as welding with a higher pressurizing force, a lower current or the same current, and a longer welding time or the same welding time as compared with the welding in the first stage, and in the third stage, welding with a higher pressurizing force and a higher current than those of the second stage is repeated (for example, refer to Patent Literature 6).
Here, spot welding of high-strength steel sheets whose tensile strength is 780 MPa or more especially has a problem that sufficient joint strength cannot be obtained because a stress concentrates on weld metal when dislocation (load) is applied to a weld and the weld metal is low in ductility and toughness. Further, the high-strength steel sheets are high in specific resistance because they contain many alloy elements, and therefore, even when the same welding current is passed therein as in the case of soft steel sheets, a heat generation amount is larger than in the case of the soft steel sheets. Further, due to their high strength, the high-strength steel sheets are more difficult to fit with electrodes than the soft steel sheets, and their contact area becomes small. In such a case, since a fusion diameter grows to a size equal to or more than a contact size between the electrodes and the steel sheets during the welding, the molten metal shoots out from an overlapping surface of the steel sheets and what is called expulsion occurs. The occurrence of such expulsion has a problem of causing reduction and variation of a size of the weld metal, that is, of a joint area, leading to deterioration also of joint strength. Further, in actual production, the fact itself that a proper current range with which a predetermined weld metal size is obtained without any occurrence of expulsion is narrow is also a problem. Therefore, in the spot welding of the high-strength steel sheets, there is generally adopted a method which increases the contact size (area) between the electrodes and the steel sheets to prevent the occurrence of expulsion by setting a pressurizing force by the electrodes high, and increases a proper current range, to thereby stably secure the joint area.
However, when the high-strength steel sheets are spot-welded while the pressurizing force of the electrodes is set high, concavities generally called indentations due to plastic deformation are generated on surfaces of the stacked high-strength steel sheets (refer to the indentations 4 in FIG. 1 and FIG. 2). The indentations (concavities) being thus too large cause a problem of lowering joint strength on the contrary even when a nugget (refer to the nugget 3 in FIG. 1 and FIG. 2) with a large diameter is obtained. Therefore, in the spot welding of the high-strength steel sheets, it has been considered to appropriately decrease the pressurizing force of the electrodes within a range enabling to obtain a predetermined joint area in order to suppress the generation of large indentations.
However, when the pressurizing force at the time of the welding is set low in order to suppress the generation of the indentations, the contact area between the electrodes and the steel sheets during an initial period of the welding reduces and a current density in a weld zone becomes high, so that expulsion is likely to occur. The expulsion is a phenomenon that part of the molten metal scatters out from the overlapping surface of the steel sheets at the time of the spot welding. The occurrence of this expulsion makes it difficult to secure the nugget diameter with a necessary size at the time of the spot welding, and sometimes promotes the generation of the indentations contrary to the intention. In such a case, there are problems that deterioration and variation of joint strength occur. Further, the expulsion sometimes adheres to the periphery of the weld zone, and when its removal is necessary, there is a problem that workability lowers.
Here, as a method to suppress the occurrence of the expulsion at the time of the spot welding, there has been proposed a method which adopts a welding pattern including a first step with a welding current, a second step of passing a welding current that is 20 to 90% of that in the first step, and a third step of further passing a welding current larger than that in the first step (for example, refer to Patent Literature 7).
Further, as a method to suppress the occurrence of the expulsion at the time of the spot welding, there has been proposed a method that adopts a welding pattern including a first step with a welding current, a second step where a welding current that is 20 to 90% of that in the first step is passed and a pressurizing force of electrodes is set larger than that in the first step, and a third step where a welding current and a pressurizing force of the electrodes are set larger than those in the first step (for example, refer to Patent Literature 8).
Further, as a method of spot-welding high-tension steel sheets whose sheet thickness is 1.8 mm or more and tensile strength is 580 MPa or more, there has been proposed a method in which a pressurizing force and a welding current value at the time of the spot welding are set according to conditions satisfying predetermined expressions, and the spot welding is performed while expulsion is generated (for example, refer to Patent Literature 9).
Further, as a method of resistance spot-welding a sheet assembly composed of two stacked thick sheets and a steel sheet with a small sheet thickness stacked on an upper surface of the two thick sheets, having sheet gaps between the steel sheets, and having a large sheet thickness ratio, there has been proposed a method in which, in an advance step, a high pressurizing force is applied to crush the sheet gaps and thereafter the pressurizing force is reduced, and in a main step, the welding is performed for a short time with a low pressurizing force and a high current in an initial period of the welding, and the welding is performed with a high pressurizing force in a latter period of the welding (for example, refer to Patent literature 10).